1. Field of the Invention
The present invention generally relates to a capacitance detecting circuit for detecting capacitance of a capacitive sensor which is employed for measuring pressure, acceleration, angular velocity or the like. More particularly, the invention is directed to an interface circuit connected to a capacitive sensor including first and second capacitors at least one of which has variable capacitance, wherein a common terminal of the first and second capacitors is connected to ground potential or a constant potential level.
2. Description of Related Art
A capacitive sensor for detecting a pressure of a fluid or acceleration and/or angular velocity of a moving object, can be manufactured by semiconductor micromachining techniques and can output an electric signal representing the fluid pressure, the acceleration, the angular velocity, etc. by detecting change or variation in the capacitance. The capacitive sensor as well as the capacitance detecting circuit therefor can provide advantages in that they can be manufactured in a miniaturized structure on a mass-production scale with high accuracy and enhanced reliability.
For having better understanding of the invention, description will be made in some detail of the technical background thereof by reference to FIG. 6 which shows in a sectional view a typical capacitive acceleration sensor which can be manufactured, for example, through a semiconductor micromachining process. Referring to the figure, the capacitive acceleration sensor includes an inertial mass member 1 supported on an anchor portion 2 by means of a cantilever 3, stationary electrodes 4 and 5 formed on a supporting member 6 so as to be positioned above and below the inertial mass member 1, respectively. The inertial mass member 1 and the stationary electrodes 4 and 5 cooperate to constitute capacitors 7 and 8, respectively, as shown in an equivalent circuit diagram of FIG. 7. The first and second capacitors 7 and 8 have respective capacitances C1 and C2 and constitute cooperatively a capacitive acceleration sensor device or element 9.
Operation of the capacitive acceleration sensor will be described. It is assumed that an inertial force due to acceleration acts on the inertial mass member 1 in the X-direction. Then, the inertial mass member 1 is caused to displace over a distance u in the X-direction in dependence on the magnitude of the acceleration. As a result of the displacement u of the inertial mass member 1, the capacitance between the inertial mass member 1 and the stationary electrode 5 increases (i.e., C+.DELTA.C, where .DELTA.C represents increment of the capacitance) while the capacitance between the inertial mass member 1 and the other stationary electrode 4 decreases (i.e., C-.DELTA.C).
As a method of converting the change in the capacitance corresponding to the displacement of the inertial mass member 1 into a voltage signal, there may be mentioned, by way of example, a switched capacitor circuit technique disclosed in Rudlf et al's "A BASIC FOR HIGH-RESOLUTION CAPACITIVE MICROACCELEROMETERS, SENSOR & ACTUATOR", A21-A23 (1990), pp. 278-281.
FIG. 8 is a circuit diagram showing a conventional or hitherto known conversion circuit for converting change of capacitance of a capacitive sensor into an electric signal, and FIG. 9 is a timing chart for illustrating operation of switches incorporated in the circuit shown in FIG. 8
Referring to FIG. 8, a capacitive sensor element 9 is implemented in a structure similar to that shown in FIG. 7 and includes the first and second capacitors 7 and 8, wherein the junction or common terminal thereof (i.e., connection node between one ends of the capacitors 7 and 8) is connected to an inversion input terminal of a first stage operational amplifier A1 incorporated in an impedance conversion circuit 10, while the other end of the first capacitor 7 is coupled to a source voltage Vs by way of a switch SW1 and additionally connected to a non-inversion input terminal of the first stage operational amplifier A1 by way of a switch SW2. On the other hand, the other end of the second capacitor 8 is connected to the ground potential (Gnd) via a switch SW4 and additionally to a conductor or wire interconnecting the switch SW2 and the non-inversion input terminal of the first stage operational amplifier A1 by way of a switch SW3. The impedance conversion circuit 10 includes a second stage operational amplifier A2 in addition to the first stage operational amplifier A1, wherein the inversion input terminal of the first stage operational amplifier A1 is connected to the common terminal of the first and second capacitors 7 and 8. The output of the first stage operational amplifier A1 is fed back to the inversion input terminal of the first amplifier A1 via a feedback capacitor 11 across which a switch SW5 is connected in parallel. The non-inversion input terminal of the first stage operational amplifier A1 is connected to a reference voltage source Vr via a holding capacitor 14. The inversion input terminal of the second stage operational amplifier A2 is connected to the output terminal of the first stage operational amplifier A1 by way of a switch SW6 and a fourth capacitor 12, while the non-inversion input terminal of the second stage operational amplifier A2 is connected to the reference voltage Vr and additionally to the fourth capacitor 12 via a switch SW7 The output of the second stage operational amplifier A2 is fed back to the inversion input terminal thereof by way of a fifth capacitor 13 and additionally to the non-inversion input terminal of the first stage operational amplifier A1 via a switch SW8.
The switches SW1, SW4 and SW6 are closed in response to a first timing clock signal .phi.1 illustrated in FIG. 9, while the switches SW2, SW3, SW5, SW7 and SW8 are closed in response to a second clock timing signal .phi.2 also illustrated in FIG. 9.
When the switches SW1 and SW4 are closed or turned on in response to the first clock timing signal .phi.1, the source voltage Vs is applied to the one terminal of the first capacitor 7 constituting a part of the capacitive sensor element 9 by way of the closed switch SW1, and the other terminal of the second capacitor 8 is coupled to the ground potential by way of the closed switch SW4. At that time point, when the junction electrode (common terminal) of the first and second capacitors 7 and 8 is not connected to the inversion input terminal of the first stage operational amplifier A1, electric charge is stored in the first and second capacitors 7 and 8, respectively, in a quantity equal to each other as a result of which there makes appearance at the common terminal of the first and second capacitors 7 and 8 a voltage Vm which can be expressed as follows: ##EQU1## where S represents a sensor sensitivity which can be expressed by EQU S=(C1-C2)/(C1+C2)
For the convenience of discussion, it is assumed that the capacitances C1 and C2 can be expressed as follows: EQU C1=Co/(1-X) EQU C2=Co/(1+X)
where X represents a relative displacement relative to the initial distance between the inertial mass member 1 and the stationary electrodes 4 and 5 (hereinafter also referred to also as the inter-electrode relative displacement), respectively. Thus, the following relation applies valid. EQU S=X
In other words, the sensitivity S of the capacitive sensor element 9 corresponds to or depends on the inter-electrode relative displacement X.
In designing the interface circuit for the capacitive sensor or sensor element 9 it is preferred to extract the voltage signal given by the above-mentioned expression (1) with low impedance to thereby generate an output voltage representing proportionally the interelectrode relative displacement X. However, because the capacitive sensor element 9 is constituted by the capacitors having the capacitances ordinarily on the order of several pico-farads (pF) to several ten pico-farads (pF) and exhibits extremely high output impedance, impedance conversion is performed by using the succeeding impedance conversion circuit 10. Basically, the impedance conversion circuit 10 is so designed as to determine the non-inversion input voltage for the first stage operational amplifier A1 so that the electric charge can be stored in the first and second capacitors 7 and 8, respectively, in an equal quantity.
The conventional conversion circuit described above suffers drawback that the input offset voltage Vos1 of the first stage operational amplifier A1 makes appearance at the output terminal after having been amplified, as can be seen from the expression (2) mentioned below. The capacitance C3 of the third feedback capacitor 11 can not be decreased excessively in order to ensure the stability of the first stage operational amplifier A1. As the sum (C1+C2) of the capacitances C1 and C2 of the first and second capacitors 7 and 8 is selected smaller when compared with the capacitance C3 of the third capacitor 11, the offset voltage Vos1 appearing at the output is amplified by a factor corresponding to the ratio between the sum capacitance (C1+C2) and the capacitance C3. EQU Vout={C1/(C1+C2)}.multidot.Vs+{C3/(C1+C2)-1}.multidot.Vos1 (2)
As a result of this, the temperature dependency of the input offset voltage Vos1 becomes more remarkable, incurring degradation of the temperature characteristic of the output of the sensor to a disadvantage.
Furthermore, in the case where the common terminal 3 of the differential capacitance type sensor element 9 is connected to the ground potential or a fixed potential level, as shown in FIG. 7, the circuit shown in FIG. 8 can not be used as it is, giving rise to a problem.
Besides, because the output of the sensor is amplified in a succeeding operational amplifier with a view enhancing the sensitivity of the sensor, the drift component ascribable to the temperature characteristic of the capacitive sensor element 9 is amplified, incurring noticeable noise in the sensor output signal and thus degrading the reliability of the sensor output signal.
Moreover, because the common terminal 3 of the capacitive sensor element 9 at which the voltage Vm given by the expression (1) makes appearance in dependence on the displacement of the inertial mass member 1 is in the floating state, difficulty will be encountered in the attempt for providing additionally an actuating electrode in opposition to the terminal 3 for thereby actuating the inertial mass member 1 by making use of an electrostatic force acting between the electrodes when a driving voltage Va is applied to the actuating electrode for the purpose of diagnosing the sensor operation. In that case, the effective interelectrode voltage is no more than a difference (Va-Vm) between the voltages Va and Vm. Consequently, sufficiently high actuating voltage can not be made use of, which in turn means that the result of the diagnosis is poor in the reliability, giving rise to a problem.